CrB2 -NbB2 /Al2 03 and CrB2 -NbB2 /SiC ceramic composite materials

ABSTRACT

A composite with a reinforcing material that is Al 2  O 3 , SiC, or xtures thereof in a matrix that is a ceramic material based on a solid solution of CrB 2  and NbB 2 .

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of copending U.S. patent application Ser. No. 08/551,253 filed on Oct. 31, 1995, now U.S. Pat. No. 5,571,759.

BACKGROUND OF THE INVENTION

This invention relates to ceramic materials and more particularly to ceramic materials based on metal borides.

Early studies on diboride materials had focused primarily on group VI metal diborides. Ceramics based on zirconium diboride (ZrB₂) and hafnium diboride (HfB₂) were developed through the 1960s by the U.S. Air Force for advanced hypersonic vehicle leading edges. While those materials are useful, it was desirable to improve their physical properties.

In U.S. Pat. No. 5,571,759 filed on Oct. 31, 1995 (parent of the present application), ceramic materials based on solid solutions of chromium diboride (CrB₂) and niobium diboride (NbB2) are disclosed which have excellent hardness and resistance to oxidation. Unfortunately, these materials have low strengths in the compositions having the highest hardness and highest resistance to oxidation. It would be desirable to produce materials having greater strength but which retain the excellent hardness and resistance to oxidation of the CrB₂ --NbB₂ ceramic materials.

SUMMARY OF THE INVENTION

Accordingly, an object of this invention is to provide a new ceramic composite material.

Another object of this invention is to provide a new ceramic composite material that is strong.

A further object of this invention is to provide a new ceramic composite material that is very hard.

Yet another object of this invention is to provide a new ceramic composite material that is resistant to oxidation.

These and other objects of this invention are achieved by providing

A ceramic composite material comprising

A. from about 5 to about 35 volume percent of a reinforcing material that is alumina (Al₂ O₃), silicon carbide (SiC), or mixtures thereof; and

B. a ceramic material that is based on a solid solution comprising from more than zero to less than 100 mole percent of CrB₂ with the remainder in the solid solution being NbB₂.

DRAWINGS

A more complete understanding of the invention and many of the attendant advantages thereto will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying figures wherein:

FIG. 1 is a graph of flexural strength and of hardness versus composition for ceramic materials based on CrB₂ --NbB₂ solid solutions;

FIG. 2 a graph of strength versus composition for ceramic materials based on CrB₂ --NbB₂ solid solutions (1) alone, (2) reinforced with 20 volume percent Al₂ O₃, and (3) reinforced with 20 volume percent SiC; and

FIG. 3 is a graph of hardness versus composition for the ceramic materials of FIG. 2.

DESCRIPTION

This application is a continuation-in-part application of copending U.S. Pat. No. 5,571,759 filed on Oct. 31, 1995 by Inna G. Talmy, Eric J. Wuchina, James A. Zaykoski, and Mark Opeka, and titled "CrB₂ --NbB₂ Ceramics Materials", hereby incorporated by reference in its entirety.

In the Ser. No. 08/551,253 application ceramic materials based on a solid solutions of CrB₂ and NbB₂ are disclosed which are very hard and resistant to oxidation at elevated temperatures. Unfortunately, it has since been found that the flexural strength of these CrB₂ --NbB₂ ceramic materials is lowest over the compositional range where hardness is greatest.

FIG. 1 is a plot of flexural strength (diamonds, ⋄) and Vickers hardness (circles, ◯) versus mole percent CrB₂ (0, 20, 40, 60, 80, and 100) in the CrB₂ --NbB₂ solid solution. Each circle (◯) is an average of the Vickers hardness measurements for that composition. The bars above and below the average hardness curve show the range of the Vickers hardness test results for each composition. Similarly, each diamond (⋄) is an average of the flexural strength measurements for that composition. The bars above and below the average flexural strength curve show the range of the flexural strength test results for each composition. FIG. 1 shows an inverse relationship been the hardness (◯) and the flexural strength (⋄) of the CrB₂ --NbB₂ solid solution ceramic materials. The compositions more desirable because of their hardness are less desirable because of their low flexural strength.

As shown in FIG. 2 and table 2, the CrB₂ ---NbB₂ /Al₂ O₃ and CrB₂ --NbB₂ /SiC ceramic composite materials of the present invention provide greater flexural strengths than the CrB₂ --NbB₂ solid solution ceramic materials alone. Moreover, as shown in FIG. 3 and table 3, the CrB₂ --NbB₂ /Al₂ O₃ and the CrB₂ --NbB₂ /SiC ceramic composite materials maintain excellent hardness. Data in Table 4 shows that CrB₂ --NbB₂ /Al₂ O₃ ceramic composite materials have higher oxidation rates at 1100° C. and 1200° C. than the unreinforced CrB₂ --NbB₂ ceramic materials. However, table 4 also shows that CrB₂ --NbB₂ /SiC ceramic composite materials have significantly lower oxidation rates than the corresponding unreinforced CrB₂ --NbB₂ ceramic materials. SiC is the preferred reinforcement material because it produces composites with (1) greater flexural strength and (2) greater resistance to oxidation at higher temperatures. Moreover, the CrB₂ --NbB₂ /SiC composites can be prepared at higher temperatures than the CrB₂ --NbB₂ /Al₂ O₃ composites.

The ceramic composite materials of the present invention preferably comprise from more than zero to about 40, more preferably from about 10 to about 30, still more preferably from 15 to 25, and even more preferably from 18 to 22 volume percent of a reinforcing material that is alumina (Al₂ O₃) or silicon carbide (SIC) or mixtures of alumina and silicon carbide with the remainder of the ceramic composite material being a matrix of a ceramic based on a solid solution of CrB₂ and NbB₂. SiC is the more preferred reinforcement material. The Al₂ O₃ and SiC reinforcement materials are preferably in the form of whiskers or powders (particles) with powders being more preferred. In the preferred embodiment Al₂ O₃ particles or SiC particles are uniformly distributed throughout the CrB₂ --NbB₂ solid solution ceramic matrix, resulting in strength being an isotropic property. The ceramic material of the matrix is based on a solid solution of chromium diboride (CrB₂) and niobium diboride (NbB₂). The CrB₂ --NbB₂ solid solution comprises from more than zero to less than 100 mole percent CrB₂ with the remainder in the solid solution being NbB₂. When the composite is used at temperatures of 1000° C. or lower, the preferred compositional ranges of the CrB₂ --NbB₂ solid solution are selected to provide hardness. For hardness, the ceramic matrix material is based on a CrB₂ --NbB₂ solid solution having a composition of preferably from about 20 to about 90, more preferably from 50 to 85, still more preferably from 60 to 85, and even more preferably from 75 to 80 mole percent of CrB₂ with the remainder in the solid solution being NbB₂. For oxidation resistance when the composite is used at temperatures about 1000° C., the ceramic matrix material is based on a CrB₂ --NbB₂ solid solution having a composition of preferably from about 15 to about 70, more preferably from 20 to about 60 mole percent CrB₂ with the remainder in the solid solution being NbB₂.

The first step in process of making the CrB₂ --NbCrB₂ /Al₂ O₃ and CrB₂ --NbB₂ /SiC₂ composites of the present invention is the preparation of a mixture of the raw materials. For the preferred embodiment in which Al₂ O₃ particles or SiC particles are uniformly distributed throughout a ceramic CrB₂ --NbB₂ solid solution matrix, the Al₂ O₃ or SiC powder, CrB₂ powder, and NbB₂ powder are uniformly and intimately mixed using conventional methods. For example, a slurry of the desired proportions of Al₂ O₃ or SiC powder, CrB₂ powder, and NbB₂ powder and a volatile organic solvent (methanol, acetone, etc.) is formed and thoroughly mixed. The powder mixture is dried and then passed through a 500 micron screen three times. For composites in which the Al₂ O₃ or SiC reinforcing material is not a powder (whiskers, fibers, etc.), an uniform, intimate mixture of the CrB₂ and NbB₂ powders is prepared and then mixed with the reinforcing material by suitable means. Whatever the form of the composite, it is necessary that the CrB₂ and NbB₂ powders are uniformly and intimately mixed so that the CrB₂ --NbB₂ solid solution ceramic material is produced.

The CrB₂ /NbB₂ /Al₂ O₃ or CrB₂ /NbB₂ /SiC is then hot pressed until the intimately mixed CrB₂ and NbB₂ powders are converted into a CrB₂ --NbB₂ solid solution which forms a matrix in which the Al₂ O₃ or SiC reinforcement material particles, whiskers, or pieces are distributed. For simple geometric shapes conventional die hot pressing techniques are used. More complex ceramic bodies can be prepared by conventional hot isostatic pressing (HIP). The hot pressing must be done in a noble gas (argon, helium, neon, etc.) atmosphere or in a vacuum.

Samples of CrB₂ --NbB₂ solid solution ceramic materials which were prepared using a graphite die are listed in table 1. Table 1 gives the compositions of the intimate mixtures of CrB2 and NbB₂ powders used, as well as the process temperature, pressure, and time for each composition.

                  TABLE 1                                                          ______________________________________                                         MATRIX COMPOSITION                                                                             PROCESS CONDITIONS                                             Mole %   Weight %   Temperature                                                                               Pressure                                                                              Time                                     NbB.sub.2                                                                            CrB.sub.2                                                                             NbB.sub.2                                                                              CrB.sub.2                                                                           (°C.)                                                                            (MPa)  (min.)                               ______________________________________                                         100    0     100     0    2100     20     30                                   95     5     96.76   3.27 2100     30     30                                   90    10     93.33   6.66 2000     30     30                                   80    20     86.15   13.84                                                                               2000     30     30                                   60    40     70.00   30.00                                                                               1900     30     30                                   40    60     50.91   49.09                                                                               1900     30     30                                   20    80     28.01   72.00                                                                               1900     30     30                                   10    90     14.74   85.35                                                                               1900     30     30                                    5    95     7.57    92.43                                                                               1900     30     30                                    0    100    0       100  1900     20     30                                   ______________________________________                                    

The melting point (˜2200° C.) of CrB₂ limits the upper end of the process temperature range. The process temperature should be at least 50 degrees below the melting point of CrB₂. The process conditions in table 1 can also be used when the CrB₂ --NbB₂ solid solution matrix is reinforced with SiC (m.p. ˜2700° C.). However, when the CrB₂ --NbB₂ solid solution matrix is reinforced with Al₂ O₃ the process temperature is preferably about 2000° C. or less because of the relatively low melting point of Al₂ O₃. Good quality CrB₂ --NbB₂ /Al₂ O₃ composites were prepared at 2000° C. For Al₂ O₃ reinforced CrB₂ --NbB₂ solid solution composed of from more than 5 to less than 100 mole percent CrB₂ with the remainder being NbB₂ in the solid solution a process temperature of 2000° C., process pressure of 30 MPa, and process time of 30 minutes works well. For Al₂ O₃ reinforced CrB₂ --NbB₂ solid solution having from more than zero to 5 mole percent CrB₂, a process temperature of 2000° C. or less can be used by increasing the process time, pressure, or both. Finally, again it should be emphasized that the hot pressing used to form either the CrB₂ NbB₂ /SiC or the CrB₂ NbB₂ /Al₂ O₃ composites must be done in a noble gas (argon, helium, neon, etc.) atmosphere or vacuum.

EXPERIMENTAL

The process of making the CrB₂ --NbB₂ solid solution ceramic materials and the Al₂ O₃ or SiC particle reinforced CrB₂ --NbB₂ solid solution ceramic composites began with the preparation of an intimate mixture of the appropriate amounts of CrB₂, NbB₂, SiC, and Al₂ O₃ powders. Commercially available CrB₂, NbB₂, SiC, and Al₂ O₃ powders having a particle size and purity suitable for ceramic processing are used. CrB₂ powders (99.5% purity, -325 mesh) and NbB₂ powders (99.5% purity, -325) mesh from CERAC INC. were used. Al₂ O₃ powders (99.9% purity, 3.5 micron average size) from Alcoa and SiC powders (98.5 % purity, 4.0 micron average size) from Electro Abrasives Corporation were used as reinforcement materials. The intimate mixture is formed by conventional means. In the examples, a slurry of the desired proportions of CrB₂ powder, NbB₂ powder, and SiC powder or Al₂ O₃ powder and a volatile organic solvent (methanol, acetone, etc.) was formed and thoroughly mixed. The CrB₂ /NbB₂ powder mixture, CrB₂ /NbB₂ /SiC powder mixture, or CrB₂ /NbB₂ /Al₂ O₃ powder mixture was dried and passed through a 500 micron screen 3 times. The intimate powder mixture was then hot pressed to form the plain ceramic or composite ceramic material.

FIG. 2 provides a comparison of the flexural strengths of unreinforced CrB₂ --NbB₂ solid solution ceramic materials (squares, □), with the CrB₂ --NbB₂ solid solution reinforced with 20 volume percent Al₂ O₃ powder (circles, ◯), or with 20 volume percent SiC powder (triangles, ⋄). The points (□,◯, Δ) represent averages of the flexural strength measurements for that composition with the bars or lines above and below the points representing the range of flexural strength measurements for that composition. Table 2 summarizes the average flexural strength measurements for each composition.

                  TABLE 2                                                          ______________________________________                                         Flexural Strength of Ceramics in the System NbB.sub.2 --CrB.sub.2,             with and without additives                                                                      Flexural Strength                                                                           Standard                                         Composition      (MPa)        Deviation                                        ______________________________________                                         100 NbB.sub.2    274.8        17.2                                             20CrB.sub.2 /80NbB.sub.2                                                                        236.6        15.6                                             40CrB.sub.2 /60NbB.sub.2                                                                        146.9        9.6                                              60CrB.sub.2 /40NbB.sub.2                                                                        139.5        14.25                                            80CrB.sub.2 /20NbB.sub.2                                                                        129.8        13.9                                             100CrB.sub.2     195.0        14.4                                             60CrB.sub.2 /40NbB.sub.2 +                                                                      261.8        31.9                                             20% Al.sub.2 O.sub.3                                                           60CrB.sub.2 /40NbB.sub.2 + 20% SiC                                                              311          14.6                                             80CrB.sub.2 /20NbB.sub.2 +                                                                      185.8        36.3                                             20% Al.sub.2 O.sub.3                                                           80CrB.sub.2 /20NbB.sub.2 + 20% SiC                                                              267.1        24.0                                             ______________________________________                                    

As can be seen from FIG. 2 and table 2, the addition of Al₂ O₃ or SiC particles substantially increases the flexural strength of the Crb₂ /NbB₂ solid solution ceramic materials. For example the flexural strength of 60 mole % CrB₂ /40 mole % NbB₂ was increased from 139.5 MPa to 261.8 MPa by the use of 20 volume % Al₂ O₃ and to 311 MPa by the use of 20 volume % SiC. Similarly, the flexural strength 80 mole % CrB₂ /20 mole % NbB₂ was increased from 129.8 MPa to 185.8 MPa by the use of 20 volume % Al₂ O₃ and to 267.1 MPa by the use of 20 volume % SiC. However, for the composites to be useful, the increases in flexural strength must be accomplished with little loss in hardness.

FIG. 3 provides a comparison of the Vickers hardnesses of unreinforced CrB₂ --NbB₂ solid solution ceramic materials (squares, □), with the CrB₂ --NbB₂ solid solution reinforced with 20 volume percent Al₂ O₃ powder (circles, ◯), or with the CrB₂ --NbB₂ solid solution reinforced with 20 volume percent SiC powder (triangles, Δ). The Vickers method of hardness testing was used with a 20 Kg load. The points (□, ◯, Δ) represent averages of the Vickers hardness measurements for that composition with the lines above and below the points representing the range of hardness measurements for that composition. Note that the CrB₂ --NbB₂ /Al₂ O₃ points (◯) and the CrB₂ NbB₂ /SiC points (AΔ) (and their associated lines) have been offset from the 60 mole percent point and the 80 more percent point in order to be more easily read. The composites (Al₂ O₃ and SiC) were made with 60 mole percent CrB₂ +40 mole percent NbB₂ and with 80 mole percent CrB₂ +20 mole percent NbB₂, however. Table 3 summaries the average Vickers hardness measurements for each composition.

                  TABLE 3                                                          ______________________________________                                                            HARDNESS*                                                   COMPOSITION        (Kg/mm.sup.2)                                               ______________________________________                                         100NbB.sub.2       1960 ± 40                                                20CrB.sub.2 /80NbB.sub.2                                                                          2340 ± 128                                               40CrB.sub.2 /60NbB.sub.2                                                                          2350 ± 150                                               60CrB.sub.2 /40NbB.sub.2                                                                          2670 ± 224                                               80CrB.sub.2 /20NbB.sub.2                                                                          3000 ± 335                                               100CrB.sub.2       1300 ± 40                                                80 Vol % 60CrB.sub.2 /40NbB.sub.2 +                                                               2500 ± 88                                                20 Vol % Al.sub.2 O.sub.3                                                      80 Vol % 60CrB.sub.2 /40NbB.sub.2 +                                                               2700 ± 101                                               20 Vol % Al.sub.2 O.sub.3                                                      80 Vol % 80CrB.sub.2 /20NbB.sub.2 +                                                               2850 ± 221                                               20 Vol % Al.sub.2 O.sub.3                                                      80 Vol % 80CrB.sub.2 /20NbB.sub.2 +                                                               2830 ± 205                                               20 Vol % SiC                                                                   ______________________________________                                          *determined by Vickers hardness test using a 20 Kg load                  

As shown by FIG. 3 and Table 3, the addition of either Al₂ O₃ or SiC particles as reinforcement material causes very little reduction in hardness of the CrB₂ /NbB₂ solid solution material.

Finally, the composite material should have good resistance to oxidation at elevated temperatures. Comparative data for the oxidation of the composites at elevated temperatures is given in table 4.

                  TABLE 4                                                          ______________________________________                                         Oxidation (2 hours) of NbB.sub.2 --CrB.sub.2 Ceramics with Al.sub.2            O.sub.3                                                                        and SiC Additions Data is Given as Weight Gain/Surface                         Area (× 10.sup.4 grams)                                                  Composition*     1100° C.                                                                         1200° C.                                      ______________________________________                                         60CrB.sub.2 /40NbB.sub.2                                                                        -2.6     11                                                   60CrB.sub.2 /40NbB.sub.2 +                                                                      23.61    177                                                  20 v % Al.sub.2 O.sub.3                                                        60CrB.sub.2 /40NbB.sub.2 +                                                                      42.57    21.6                                                 20 v % SiC                                                                     80CrB.sub.2 /20NbB.sub.2                                                                        162      469                                                  80CrB.sub.2 /20NbB.sub.2 +                                                                      301      675                                                  20 v % Al.sub.2 O.sub.3                                                        80CrB.sub.2 /20NbB.sub.2 +                                                                      73.7     64                                                   20 v % SiC                                                                     ______________________________________                                          *materials were hot pressed at 1900° C.                           

For temperatures below 1000° C. the Al₂ O₃ -reinforced and the SiC-reinforced CrB₂ --NbB₂ solid solution ceramic materials are resistant to oxidation as are unreinforced CrB₂ --NbB₂ solid solution ceramic materials. Table 4 shows that at temperatures of 1100° C. and 1200° C. the Al₂ O₃ -reinforced CrB₂ --NbB2 solid solution ceramic material are more susceptible to oxidation and the SiC-reinforced solid solution ceramic materials are less susceptible to oxidation than the corresponding unreinforced materials.

For maximum hardness with good flexural strength and low rates of oxidation at elevated temperatures, the preferred embodiment is a composite based on 20 volume percent silicon carbide (SiC) particles uniformly distributed in 80 volume percent of a matrix of a solid solution of 80 mole percent CrB₂ and 20 mole percent of NbB₂. The composite is preferably formed by hot pressing at 2000° to 2100° C. in an inert atmosphere (argon, helium, neon, etc.) or in vacuum.

Obviously, other modifications and variations of the present invention may be possible in light of the foregoing teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described. 

What is claimed is:
 1. A composite comprisingA. from more than zero to about 40 volume percent of a reinforcement material that is Al₂ O₃, SiC, or mixtures thereof; and B. the remainder of the composite being a matrix of a ceramic material based on a solid solution comprising from more than zero to less than 100 mole percent CrB₂ with NbB₂ being the remainder in solid solution.
 2. The composite of claim 1 wherein the reinforcement material is in the form of a powder.
 3. The composite of claim 2 wherein the reinforcement material is uniformly distributed throughout the solid solution of CrB₂ and NbB₂.
 4. The composite of claim 1 wherein the reinforcement material is Al₂ O₃.
 5. The composite of claim 1 wherein the reinforcement material is SiC.
 6. The composite of claim 1 wherein the reinforcement material comprises from about 10 to about 30 volume percent of the composite.
 7. The composite of claim 6 wherein the reinforcement material comprises from 15 to 25 volume percent of the composite.
 8. The composite of claim 7 wherein the reinforcement material comprises from 18 to 22 volume percent of the composite.
 9. The composite of claim 1 wherein the solid solution comprises from about 20 to about 90 mole percent of CrB₂ with NbB₂ being the remainder in the solid solution.
 10. The composite of claim 9 wherein the solid solution comprises from 50 to 85 mole percent of CrB₂ with NbB₂ being the remainder in the solid solution.
 11. The composite of claim 10 wherein the solid solution comprises from 60 to 85 mole percent of CrB₂ with NbB₂ being the remainder in the solid solution.
 12. The composite of claim 11 wherein the solid solution comprises from 75 to 80 mole percent of CrB₂ with NbB₂ being the remainder in solid solution.
 13. The composite of claim 1 wherein the solid solution comprises from 15 to 70 mole percent of CrB₂ with NbB₂ being the remainder in solid solution.
 14. The composite of claim 13 wherein the solid solution comprises from 20 to 60 mole percent of CrB₂ with NbB₂ being the remainder in solid solution.
 15. The composite of claim 5 wherein the reinforcement material is SiC powder.
 16. The composite of claim 15 wherein the SiC powder is uniformly distributed throughout the solid solution of CrB₂ and NbB₂.
 17. The composite of claim 5 wherein the reinforcement material comprises from about 10 to about 30 volume percent of the composite.
 18. The composite of claim 17 wherein the reinforcement material comprises from 15 to 25 volume percent of the composite.
 19. The composite of claim 18 wherein the reinforcement material comprises from 18 to 22 volume percent of the composite.
 20. The composite of claim 5 wherein the solid solution comprises from about 20 to about 90 mole percent of CrB₂ with NbB₂ being the remainder in the solid solution.
 21. The composite of claim 20 wherein the solid solution comprises from 50 to 85 mole percent of CrB₂ with NbB₂ being the remainder in the solid solution.
 22. The composite of claim 21 wherein the solid solution comprises from 60 to 85 mole percent CrB2 with NbB₂ being the remainder in the solid solution.
 23. The composite of claim 22 wherein the solid solution comprises from 75 to 80 mole percent of CrB₂ with NbB₂ being the remainder in solid solution.
 24. The composite of claim 5 wherein the solid solution comprises from 15 to 70 mole percent CrB₂ with NbB₂ being the remainder in solid solution.
 25. The composite of claim 24 wherein the solid solution comprises from 20 to 60 mole percent of CrB₂ with NbB₂ being the remainder in the solid solution. 